Traction control systems help to prevent or limit a vehicle's wheels from slipping during acceleration on different surfaces. Traction of a vehicle is established as its wheels contact a surface so that when the wheels are rotated, usually by a driving force, the vehicle will be moved along the surface in a desired direction. The combination of the coefficient of friction and the force exerted by a wheel against the surface produces traction. When the coefficient of friction of the surface is less than the force exerted, the wheel will slip during acceleration of the vehicle, adversely affecting acceleration performance and driving stability. Slippage can occur as a result of excessive accelerative forces applied to vehicle wheels or inadequate wheel to surface friction that can be present with wet or icy conditions. Once the condition is recognized, a vehicle driver, particularly in an automobile or like vehicle, may try to control slippage by reducing engine power or by applying the brakes, both of which can reduce the speed at which a drive wheel is rotating. The driver may not be immediately aware that slippage is occurring, however, and may not be able to take corrective action as quickly as required.
Various traction control systems have been proposed for automobiles and like vehicles to automatically adjust traction between the vehicle's drive wheels and the road or ground surface to minimize acceleration slip. These include, for example, systems that control traction using braking force adjustment, engine throttle control, and engine fuel supply control. Other traction control systems for automotive use have also been proposed. In U.S. Pat. No. 6,002,979 to Ishizu (Nissan), for example, an automobile traction control system in combination with a fuel supply system that adjusts driving torque delivered to each drive wheel by adjusting engine power is described. This system monitors slipping of a drive wheel with respect to a target drive wheel speed and includes engine control means that cooperates with a fuel supply system to decrease engine power by decreasing fuel supplied in response to a detected slipping condition. This system is sensitive to preventing engine stall when the speed of the drive wheel is reduced to a target drive wheel speed. A plurality of sensors is employed to assist with the electronic control of the Ishizu traction control system.
The traction control device disclosed in U.S. Pat. No. 6,007,454 by Takahira et al (Toyota) automatically detects slipping conditions of each of the pairs of wheels in a four wheel drive automobile by comparing the mean rotational speed of the front or rear drive wheels to a threshold value. A brake system is electronically controlled to apply brakes to at least one of the pairs of front or rear wheels, thereby executing traction control according to a selected gear transmission ratio. When optional vehicle speed sensors are included in this system and wheel rotation speeds are compared to vehicle speed, the automobile's engine can be controlled to decrease rotational power output. Neither of the systems described in the aforementioned patents would effectively control traction in a vehicle that is powered solely by an electric drive motor or other electric drive means
Traction control systems useful in hybrid and electric vehicles, primarily automobiles, have also been proposed. U.S. Pat. No. 5,450,324 to Cikanek (Ford), for example, discloses a combined traction control and antiskid braking system operatively connected to an electric traction motor and a hydraulic braking system. Present vehicle parameters are monitored by sensors, and a processor responsive to the sensors calculates vehicle parameters not directly measurable to determine whether the vehicle state requires traction control or braking control. A control strategy based on the determined vehicle state is used by the processor to provide commands to a motor controller to control operation of the electric traction motor by reducing motor torque if traction control is appropriate or, alternatively, to a brake controller if hydraulic or regenerative antiskid braking control is needed. The main focus of the traction control and braking system disclosed by Cikanek is to maximize regenerated kinetic energy during braking and minimize kinetic energy loss due to wheel slip, primarily to overcome battery energy storage limitations. The use of an electric drive motor or other electric drive means to control traction is not suggested.
The traction control system described in U.S. Pat. No. 6,577,944 by Davis uses existing engine speed sensors to determine the occurrence and degree of wheel slippage by comparing whether two successive engine speed readings exceed a selected threshold and generates an automatic proportional corrective action from the vehicle's engine, braking system, or both. This system is designed primarily for automotive internal combustion engines and/or electric motors, although some limited non-automotive uses in which a drive unit applies torque to a rotating component that must overcome resistance are suggested. These include a turbine rotated by an electric drive motor and a power boat with an internal combustion engine-driven propeller or screw. It is not suggested that speed measurements could be eliminated or that torque output of a drive unit or motor in the absence of speed measurements could be controlled to control traction or that traction could be controlled in an aircraft drive wheel.
A traction control system and method that eliminates the use of an electric vehicle's antilock braking system to perform traction control is described by Young in U.S. Pat. No. 5,758,014. A power inverter or controller uses rotor speed to control torque output of the electric vehicle's motor and thereby control traction. An encoder coupled to the rotor provides signals that indicate rotor speed. Alternatively, the encoder could be replaced by wheel sensors. An algorithm monitors the rate of change of the rotor speed as an indication of vehicle speed and compares the rate of change of rotor speed to a programmable threshold value to determine wheel slippage. A torque command from a controller controls the amount of current applied to the motor and the torque generated by the motor. When a loss of traction is experienced, reducing torque decreases the rotor speed and allows the vehicle's drive wheels to regain traction. The commanded torque value is based on the vehicle accelerator pedal position or cruise control torque. It is not suggested that the described speed measurements could be eliminated or that electrical parameters affecting current of an electric motor could be used to directly control traction by maintaining a maximum supply of current in a response to a requested or commanded speed.
A need exists, therefore, for a traction control system and method of controlling traction in an electric vehicle that relies on direct control of the current supply to an electric motor driving the vehicle wheels to maintain a maximum current supply for a vehicle operator commanded speed and control traction.